The present invention relates to an ink-jet recording head having piezoelectric layers formed on a surface of an elastic sheet which forms part of pressure generating chambers communicating with nozzle orifices from which ink drops are allowed to issue by displacement of the piezoelectric layers.
The operating principle of ink-jet recording heads is such that the elastic sheet described above is displaced by means of piezoelectric vibrators to apply pressure to the ink in pressure generating chambers, thereby ejecting ink drops from nozzle orifices. Practically, ink-jet recording heads are classified as one of two types depending on the piezoelectric vibrator used; one type uses a vibrator of a longitudinally vibrating mode which extends and contracts along its own axis and the other type uses a vibrator of a flexing or flexural vibrating mode.
The first type of ink-jet recording heads is capable of changing the volume of each pressure generating chamber by contacting an end face of the piezoelectric vibrator with the elastic sheet and has the advantage of being suitable for high-density printing. On the other hand, the manufacturing process of this type of head is complicated since it involves not only a difficult step of segmenting the piezoelectric elastic sheet into a combtooth-shaped pattern in registry with the pitch on which nozzle orifices are arranged but also the step of fixing the individual piezoelectric vibrators in an appropriate positional relationship with the respective pressure generating chambers.
In contrast, the second type of ink-jet recording heads has the advantage of enabling the piezoelectric vibrators to be mounted on the elastic sheet by a relatively simple process in which a green sheet of piezoelectric material is attached to a substrate is conformity with the shape of individual pressure generating chambers and baked. On the other hand, a certain area is required to permit flexural vibrations and this introduces difficulty in achieving high-density arrangement of piezoelectric vibrators.
To deal with these problems, it has been proposed as in Unexamined Published Japanese Patent Application No. Hei. 5-286131 that a uniform layer of piezoelectric material be formed over the entire surface of the elastic sheet by film deposition techniques and that the formed piezoelectric layer be segmented into shapes that correspond to the pressure generating chambers by lithographic techniques such that the piezoelectric vibrator formed in one pressure generating chamber is independent of the vibrator formed in another pressure generating chamber.
This proposal eliminates the need to attach the piezoelectric vibrators onto the elastic sheet and offers the advantage of not only enabling the piezoelectric vibrators to be mounted by the precise and yet simple lithographic techniques but also reducing the thickness of each piezoelectric vibrator by a sufficient amount to permit fast driving.
On the other hand, the piezoelectric layer is so thin that compared to the attached type of piezoelectric vibrator, the rigidity is small enough to increase the chance of stress concentration near the boundaries of each pressure generating chamber and this causes the disadvantage of shortening the life of the elastic sheet, piezoelectric vibrators and even the electrodes.
As another problem, the piezoelectric constant is only about a third to half of the value for the piezoelectric vibrator that is formed by baking an attached green sheet and this requires driving at high voltage; then, both the upper and lower electrodes will experience surface discharge along the lateral sides of the piezoelectric layer which increases the chance of a leakage current of flowing between the two electrodes, thereby destabilizing the issuance of ink droplets. A further problem is that if the piezoelectric vibrator is segmented or divided in correspondence with individual pressure generating chambers, the areas of lateral sides that are exposed to air atmosphere are increased so that the individual piezoelectric vibrators are prone to deteriorate due to the moisture in air atmosphere.